Early implantable medical devices such as implantable cardiac pacemakers were designed to operate in a typically single operating mode governed by fixed operating parameters without any ability to change the operating mode or otherwise communicate percutaneously with external equipment. In time, it became apparent that it would be clinically desirable to vary certain of the operating parameters and/or modes of operation. An initial approach employed with implanted cardiac pacemakers involved use of miniature rheostats that could be directly accessed by a needle-like tool inserted through the patient's skin to adjust a resistance in the pacing rate or pulse width setting circuit. Later, miniaturized reed switches were incorporated into the pacing rate or pulse width circuits that responded to magnetic fields applied through the skin by an external magnet placed over the implant site. The pulse width, pacing rate and a limited number of pacing modes could be adjusted in this manner.
It was also realized that the operation of an implantable cardiac pacemaker could be observed, for example, by use of a standard ECG machine and timing of intervals between pacing pulse spikes in the ECG tracing recorded from skin electrodes on the patient. Later, it became known that this technique could be used to detect data sent from the implanted cardiac pacemaker by modulating the pacing pulse amplitude and/or width or the pacing rate. This approach could only provide a low bandpass data channel, of course, to avoid interfering with the primary function of pacing the patient's heart when necessary. One use of this technique was to monitor impending battery depletion through observation of a change in the pacing rate from a preset or programmed pacing rate in response to a battery voltage drop.
As digital circuit technology advanced, it was recognized that control of operating modes and parameters of implanted medical devices could be realized in digital or binary circuits employing memorized control states or operating parameter values. In order to change an operating mode or parameter value, "programmers" were developed based on radio frequency (RF) downlink data communication from an external programmer transceiver to a telemetry transceiver and memory incorporated within the pacemaker implantable pulse generator (IPG).
It also became possible to provide uplink data telemetry to transmit the contents of a register or memory within the IPG to the telemetry receiver within the programmer employing the same RF transmission capabilities. Today, both analog and digital data can be transmitted by uplink RF telemetry from the implanted device to the external programmer. The analog data has typically included battery voltage, sampled intracardiac electrocardiogram amplitude values, sensor output signals, pacing pulse amplitude, energy, and pulse width, and pacing lead impedance. The digital data typically includes, statistics related to performance, event markers, current values of programmable parameters, implant data, and patient and IPG identifier codes. Similar analog and digital data is telemetered from implanted cardiac monitors, drug dispensers, nerve stimulators, cardioverter/defibrillators, pacemaker/cardioverter/defibrillators, etc.
The telemetry transmission system that evolved into current common use relies upon the generation of low amplitude magnetic fields by current oscillating in an LC circuit of an RF telemetry antenna in a transmitting mode and the sensing of currents induced a closely spaced RF telemetry antenna in a receiving mode. Short duration bursts of the carrier frequency are transmitted in a variety of telemetry transmission formats. In the MEDTRONIC.RTM. product line, the RF carrier frequency is set at 175 kHz and the RF telemetry antenna of the implantable medical device is typically coiled wire wound about a ferrite core that is located within the hermetically sealed enclosure. The hermetically sealed enclosure also typically contains a battery power source and circuitry for controlling the operation of the medical device.
In an uplink telemetry transmission from an implanted medical device, it is desirable to limit the current drain from the implanted battery as much as possible simply to prolong device longevity. However, as device operating and monitoring capabilities multiply, it is desirable to be able to transmit out ever increasing volumes of data in real time or in as short a transmission time as possible with high reliability and immunity to spurious noise. As a result of these considerations, many RF telemetry transmission data encoding schemes have been proposed or currently are used that attempt to increase the data transmission rate.
An extensive description of the historical development of uplink and downlink telemetry transmission formats and is set forth in the above-referenced '851 and '963 applications and in the following series of commonly assigned patents all of which are incorporated herein by reference in their entireties. Commonly assigned U.S. Pat. No. 5,127,404 to Grevious et al. sets forth an improved method of frame based, pulse position modulated (PPM) of data particularly for uplink telemetry. The frame-based PPM telemetry format increases bandwidth well above simple PIM or pulse width modulation (PWM) binary bit stream transmissions and thereby conserves energy of the implanted medical device. Commonly assigned U.S. Pat. No. 5,168,871 to Grevious et al. sets forth an improvement in the telemetry system of the '404 patent for detecting uplink telemetry RF pulse bursts that are corrupted in a noisy environment. Commonly assigned U.S. Pat. No. 5,292,343 to Blanchette et al. sets forth a further improvement in the telemetry system of the '404 patent employing a hand shake protocol for maintaining the communications link between the external programmer and the implanted medical device despite instability in holding the programmer RF head steady during the transmission. Commonly assigned U.S. Pat. No. 5,324,315 to Grevious sets forth an improvement in the uplink telemetry system of the '404 patent for providing feedback to the programmer to aid in optimally positioning the programmer RF head over the implanted medical device. Commonly assigned U.S. Pat. No. 5,117,825 to Grevious sets forth an further improvement in the programmer RF head for regulating the output level of the magnetic H field of the RF head telemetry antenna using a signal induced in a sense coil in a feedback loop to control gain of an amplifier driving the RF head telemetry antenna. Commonly assigned U.S. Pat. No. 5,562,714 to Grevious sets forth a further solution to the regulation of the output level of the magnetic H field generated by the RF head telemetry antenna using the sense coil current to directly load the H field. Commonly assigned U.S. Pat. No. 5,354,319 to Wybomey et al. sets forth a number of further improvements in the frame based telemetry system of the '404 patent. Many of these improvements are incorporated into MEDTRONIC.RTM. Model 9760, 9766 and 9790 programmers. These improvements and the improvements described in the above-referenced pending patent applications are directed in general to increasing the data transmission rate, decreasing current consumption of the battery power source of the implantable medical device, and increasing reliability of uplink and downlink telemetry transmissions.
The current MEDTRONIC.RTM. telemetry system employing the 175 kHz carrier frequency limits the upper data transfer rate, depending on bandwidth and the prevailing signal-to-noise ratio. Using a ferrite core, wire coil, RF telemetry antenna results in: (1) a very low radiation efficiency because of feed impedance mismatch and ohmic losses; 2) a radiation intensity attenuated proportionally to at least the fourth power of distance (in contrast to other radiation systems which have radiation intensity attenuated proportionally to square of distance); and 3) good noise immunity because of the required close distance between and coupling of the receiver and transmitter RF telemetry antenna fields.
These characteristics require that the implantable medical device be implanted just under the patient's skin and preferably oriented with the RF telemetry antenna closest to the patient's skin. To ensure that the data transfer is reliable, it is necessary for the patient to remain still and for the medical professional to steadily hold the RF programmer head against the patient's skin over the implanted medical device for the duration of the transmission. If the telemetry transmission takes a relatively long number of seconds, there is a chance that the programmer head will not be held steady. If the uplink telemetry transmission link is interrupted by a gross movement, it is necessary to restart and repeat the uplink telemetry transmission. Many of the above-incorporated, commonly assigned, patents address these problems.
The ferrite core, wire coil, RF telemetry antenna is not bio-compatible, and therefore it must be placed inside the medical device hermetically sealed housing. The typically conductive medical device housing adversely attenuates the radiated RF field and limits the data transfer distance between the programmer head and the implanted medical device RF telemetry antennas to a few inches.
In U.S. Pat. Nos. 4,785,827 to Fischer, 4,991,582 to Byers et al., and commonly assigned 5,470,345 to Hassler et al. (all incorporated herein by reference in their entireties), the metal can typically used as the hermetically sealed housing of the implantable medical device is replaced by a hermetically sealed ceramic container. The wire coil antenna is still placed inside the container, but the magnetic H field is less attenuated. It is still necessary to maintain the implanted medical device and the external programming head in relatively close proximity to ensure that the H field coupling is maintained between the respective RF telemetry antennas.
Attempts have been made to replace the ferrite core, wire coil, RF telemetry antenna in the implantable medical device with an antenna that can be located outside the hermetically sealed enclosure. For example, a relatively large air core RF telemetry antenna has been embedded into the thermoplastic header material of the MEDTRONIC.RTM. Prometheus programmable IPG. It is also suggested that the RF telemetry antenna may be located in the IPG header in U.S. Pat. No. 5,342,408. The header area and volume is relatively limited, and body fluid may infiltrate the header material and the RF telemetry antenna.
In U.S. Pat. Nos. 5,058,581 and 5,562,713 to Silvian, incorporated herein by reference in their entireties, it is proposed that the elongated wire conductor of one or more medical lead extending away from the implanted medical device be employed as an RF telemetry antenna. In the particular examples, the medical lead is a cardiac lead particularly used to deliver energy to the heart generated by a pulse generator circuit and to conduct electrical heart signals to a sense amplifier. A modest increase in the data transmission rate to about 8 Kb/s is alleged in the '581 and '713 patents using an RF frequency of 10-300 MHz. In these cases, the conductor wire of the medical lead can operate as a far field radiator to a more remotely located programmer RF telemetry antenna. Consequently, it is not necessary to maintain a close spacing between the programmer RF telemetry antenna and the implanted cardiac lead antenna or for the patient to stay as still as possible during the telemetry transmission.
However, using the medical lead conductor as the RF telemetry antenna has several disadvantages. The radiating field is maintained by current flowing in the lead conductor, and the use of the medical lead conductor during the RF telemetry transmission may conflict with sensing and stimulation operations. RF radiation losses are high because the human body medium is lossy at higher RF frequencies. The elongated lead wire RF telemetry antenna has directional radiation nulls that depend on the direction that the medical lead extends, which varies from patient to patient. These considerations both contribute to the requirement that uplink telemetry transmission energy be set artificially high to ensure that the radiated RF energy during the RF uplink telemetry can be detected at the programmer RF telemetry antenna. Moreover, not all implantable medical devices have lead conductor wires extending from the device.
A further U.S. Pat. No. 4,681,111 to Silvian, incorporated herein by reference in its entirety, suggests the use of a stub antenna associated with the header as the implantable medical device RF telemetry antenna for high carrier frequencies of up to 200 MHz and employing phase shift keying (PSK) modulation. The elimination of the need for a VCO and a bit rate on the order of 2-5% of the carrier frequency or 3.3-10 times the conventional bit rate are alleged.
At present, a wide variety of implanted medical devices are commercially released or proposed for clinical implantation. Such medical devices include implantable cardiac pacemakers as well as implantable cardioverter-defibrillators, pacemaker-cardioverter-defibrillators, drug delivery pumps, cardiomyostimulators, cardiac and other physiologic monitors, nerve and muscle stimulators, deep brain stimulators, cochlear implants, artificial hearts, etc. As the technology advances, implantable medical devices become ever more complex in possible programmable operating modes, menus of available operating parameters, and capabilities of monitoring increasing varieties of physiologic conditions and electrical signals which place ever increasing demands on the programming system.
It remains desirable to minimize the time spent in uplink telemetry and downlink transmissions both to reduce the likelihood that the telemetry link may be broken and to reduce current consumption.
Moreover, it is desirable to eliminate the need to hold the programmer RF telemetry antenna still and in proximity with the implantable medical device RF telemetry antenna for the duration of the telemetry transmission. As will become apparent from the following, the present invention satisfies these needs.